1. Field of the Invention
The present invention relates to power line communications, and more particularly, to configurations of data couplers for power line communications.
2. Description of the Related Art
A data coupler for power line communications couples a data signal between the power line and a communication device such as a modem. The data coupler exhibits a cutoff frequency. Below the cutoff frequency, coupling attenuation between the power line and the communication device becomes excessive. The data coupler may be either an inductive coupler or a capacitive coupler.
An inductive coupler for a power line should ideally have a magnetization inductance with an impedance that is large as compared to an impedance of the communication device. As the magnetization inductance is in shunt with a signal and inductively loads the signal, a low magnetization inductance is undesirable.
A capacitive coupler may be efficient for use on a power line, especially a low voltage line.
FIG. 1 is a schematic of a prior art capacitive coupler as may be used to couple a power line modem to a secondary power line. A power line 400, nominally neutral, is connected to a shield terminal of a coax connector 420. A power line 405, nominally an energized phase line, is connected to a center conductor contact of connector 420 via a fuse 415 and a capacitor 410. A modem 435 is connected, via a cable 430 and a connector 425, to connector 420. The capacitive coupler thus couples high frequency signals between modem 435 and power lines 400 and 405.
The value of capacitor 410 is several nanofarads, large enough to have negligible reactance at signal frequency and small enough to have a large reactance at power frequency.
A ceramic capacitor with an appropriate dielectric may be used in such a coupler and provides a low impedance to signal frequencies in the MHz range. However, a lead connecting such a capacitor to the power line may be relatively long, and may have an impedance far exceeding the capacitor's impedance at signal frequencies.
For example, wires in a low voltage rack arrangement are typically spaced 20 centimeters (cm) apart, and a 10 AWG wire of that length has an inductance of 0.21 microhenry (uH) or nearly 40 ohms at 30 megahertz (MHz). Should the capacitive coupler need to connect to a non-adjacent wire, the impedance could increase to 80 or 120 ohms. As a typical capacitive coupler must be fused, and a fuse impedance is added in series, a total series inductive impedance can be significant in comparison to a typical modem's 50 ohm impedance.